Modern vehicles require fuel management systems that control fuel vapor venting from a vehicle fuel tank to limit fuel filling and that control fuel tank ventilation to prevent overpressure and vacuum conditions in the fuel tank. Fuel vapor can be created in the fuel tank by temperature differences between the fuel tank and liquid fuel from a fuel pump, as well as by sloshing and agitation of the fuel tank during normal vehicle operation. The pressure buildup resulting from the creation of new fuel vapors must be relieved properly. For this reason, many fuel management systems are equipped with tank venting control assemblies capable of discharging a relatively large amount of fuel vapor in response to the development of high-pressure conditions in the fuel tank.
In addition to providing for adequate fuel vapor discharge from the fuel tank during high tank pressure conditions, well-designed tank pressure control assemblies must be capable of responding to a reduction of pressure in the fuel tank to or below a predetermined level. Introducing ambient air into the fuel tank to bring the fuel vapor pressure in the fuel tank back to approximately atmospheric pressure usually relieves these tank vacuum conditions.
In addition to controlling vapor escape, well-designed “On-Board Refueling Vapor Recovery” systems or (ORVR) systems assist in controlling the amount of liquid fuel that can be pumped into the fuel tank during refueling. For safety reasons, fuel systems are designed so that the fuel tank is never completely filled with liquid fuel. Rather, at least a predetermined portion of the space inside the fuel tank is left for liquid fuel and fuel vapor expansion. Although fuel pump nozzles typically include sensors for shutting off the flow of liquid fuel into the fuel tank when the fuel tank is nearly filled, fuel pump users may manually override the sensors by continuing to pump fuel after the sensors have automatically and temporarily shut the pump nozzle off. To assist in preventing tank overfill under such conditions, an ORVR system is usually provided with a “fill-limit” control system, which assists in triggering the nozzle shut-off mechanism when the level of liquid fuel in the fuel tank has risen to a predetermined level.
Conventional valve assemblies designed to meet the foregoing requirements include pressure-relief, over-fill, and/or rollover components. Typically, the valve assemblies include a sealing element made of a plastic, hard rubber, or other elastomeric material. The conventional sealing element presses against a portion of the valve assembly under fuel vapor pressure, gravity, or the like in order to seal the valve assembly. Although undesirable, the sealing element is more often than not in contact with the valve assembly due to vehicle vibrations and fuel sloshing. Over time, the recurring sealing action and repeated exposure to vehicle vibrations and fuel tank pressures can flatten and wear down edges of an elastomeric sealing element. Moreover, that portion of the valve assembly contacted and sealed by the sealing element can prematurely deteriorate. For instance, the contacted portion of the conventional valve assembly will often develop an unwanted indentation due to the repeated contact with the sealing element.
Eventual deformation of the sealing element and the contacted portion of the valve assembly adversely affects the sealing capacity of the valve assembly. It will be appreciated, for instance, that impaired sealing can degrade the rollover function of the vehicle fuel system. Specifically, a deformed sealing element in a rollover situation can permit fuel to leak past an outlet of the valve assembly to a fuel vapor recovery canister or directly to the atmosphere external to the vehicle, which may create a fire or explosion hazard.
A fill-limit and tank ventilation control system is needed that vents fuel vapor from the vapor space in a fuel tank during early stages of refueling but blocks introduction of any and all liquid fuel in excess of a maximum volume so as to preserve a minimum volume of the vapor space in the fuel tank once the fuel tank is filled to its maximum capacity with fuel. Ideally, this fill-limit and tank ventilation control system could be included as one component in a comprehensive ORVR vehicle fuel system to manage fuel vapor recovery during all phases of vehicle use. One attempt to address this requirement is disclosed in U.S. patent application Ser. No. 10/260,722, filed Sep. 30, 2002, now U.S. Pat. No. 6,758,235. However, a relatively uncomplicated ORVR system design is needed in which the sealing elements are in a spaced-apart arrangement such that the sealing elements touch only during specific fuel conditions to prevent undesirable wear and tear on the sealing elements.